Conformational Effects on the Photoinduced Energy Transfer in a Star‐Shaped Pentaporphyrin with Nucleosidic Linkers

Abstract

Abstract The photophysical properties of a pentameric array ( FB‐Zn 4 ) containing peripheral Zn porphyrins and a free‐base core, connected by nucleosidic linkers, have been determined and compared with the properties of the model dyad ( FB‐Zn ) and of their individual components. The flexibility of the nucleosidic linkers allows the dyad to take two different conformations corresponding to a bent and an almost fully extended form of the linker, which are characterized by a chromophore separation of ca. 2 nm and ca. 3 nm, respectively. Energy transfer from the zinc porphyrin to the free‐base porphyrin occurs, in the bent conformation, with a rate of 3.6 × 10 9 s −1 . Conversely, in the extended conformation, the Zn porphyrin decay is unperturbed, which is in good agreement with the expected dipole−dipole (Förster) energy transfer mechanism. In the pentameric array, the Zn porphyrin luminescence is quenched with a rate of 3.6 × 10 9 s −1 in 15% of the population, whilst the luminescence of the Zn porphyrin is unquenched for ca. 35% of the population. For comparison with the dyad, these two cases are assigned as arrays with a single linkage in a bent configuration and arrays with all linkages in the extended configuration, respectively. However, the most striking feature in the pentaporphyrin is that 50% of the Zn porphyrin excited state population exhibits a fast non‐radiative deactivation (rate ca. 10 10 s −1 ) which also involves the free‐base porphyrin core to some extent. This behavior is attributed to a strongly interacting conformation with the Zn porphyrins folded over the central free‐base porphyrin. The proposed model is supported by CD spectroscopy, complexation experiments with bases, and the reactivity of the excited state towards external quenchers. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)